Photoconductive target



1954 P. K. WEIMER PHOTOCONDUCTIVE TARGET Filed Feb. 24, 1951 007/ 07 SIG/I691 non QZIHEI P321] mm a.

*zaoK I Patented Aug. 24, 1954 2,687,484 PHOTOGONDUCTIVE TARGET Paul K. Weimer, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application February 24, 1951, Serial No. 212,550

12 Claims.

This invention is directed to photo-sensitive electrode and more specifically to an improved photoconductive tar et electrode for a pickup or camera tube for television.

A photoconductive pickup tube is one having a target formed with a supporting transparent sheet portion, which is first coated with a conductive film or signal plate and then, secondly, with a layer of known photoconductive material over and in contact with the conductive film. This target electrode is mounted within an evacuated envelope with the photoconductive coating facing an electron gun structure, which produces an electron beam substantially normal to the target surface. Either electrostatic fields or electromagnetic fields can be used to cause the electron beam to scan, in closely spaced parallel lines, over the surface of the photoconductive target layer. The beam electrons are slowed down so that they approach the target surface with low energies. Electrons from the beam are deposited on the photoconductive surface to drive the surface to substantially cathode potential. At this potential the remaining portion of the electron beam is reflected back toward the gun. There is applied to the conductive signal plate of the target, a potential that is several volts different from the cathode potential established on the photoconductive surface. In this manner, then, a difference of potential is established between the two surfaces of the photoconductive film. Due to the photoconductive properties of the target material used, when light is focused upon the photoconductive film, a current flow will take place across the film in the illuminated areas and will change these areas toward the potential of the conductive film. Areas of the target not illuminated by the light will have little or no current flow depending upon the resistivity of the photoconductive material in the dark and will thus remain at the cathode potential established by the beam. The electron beam upon scanning over the target areas illuminated by light, will return the illuminated target areas to cathode potential. Since the signal plate is capacitively coupled with the scanned surface of the target, the instantaneous charging of the target by the beam to cathode potential will be evidenced by a voltage change in the circuit of the signal plate. This voltage change becomes the output signal of the tube.

A photoconductive material, in order to be eifectively used for a pickup tube target, must exhibit sufilcient dark resistivity to give storage during tube operation as the potentials, established on the photoconductive surface areas of the target by the electron beam, must be maintained in the unilluminated target areas. If the photoconductive target material has low dark resistivity, the dark current through the material will discharge the target in the dark areas, thus, masking the light output signal derived from the illuminated areas.

The use of an amorphous selenium in pickup tubes has been successfully carried out and is fully described in my copending application Serial No. 78,687, filed February 28, 1949, which is now U. S. Patent 2,654,853 issued October 6, 1953. Amorphous selenium is a material having good sensitivity for phototube applications, as Well as, a satisfactory resistivity in the dark. The spectral response of amorphous selenium is in the blue and green with particularly no red response. Antimony trisulfide is an example of a photoconductive material, which in its usual form has a lower dark resistivity than is desirable for pickup tube applications. Although the resistivity of this material is not so low as to prevent storage, it is sumciently low that an excessive amount of beam current is used solely for discharging the current which flows in the dark.

Photoconductive pickup tube targets must be formed from a thin film of antimony trisulfide to provide sufiicient sensitivity for pickup tube operation. A thick target of antimony sulfide has low sensitivity because the charge carriers have a very short range. In a thinner target, the sensitivity is enhanced because the light is able to penetrate the entire thickness of the target and release the charges which would otherwise become trapped within the target. However, such a thin target film also provides a high capacity between its exposed surface and the signal plate, so that the electron beam cannot fully discharge the stored signals during a single scan. This results in a time lag noticeable in the transmitted picture. Also antimony trisulfide has an inherent photoconductive time-lag, which causes objectionable trailing of moving objects in the transmitted picture. A photoconductive pickup tube target made of antimony trisulfide by evaporation in vacuum does show very good red response. In color television applications, as well as in applications where the illumination is mostly in the red region of the spectrum, it is desirable to have a photoconductive tube sensitive to red light.

It is therefore an object of my invention to provide a photoconductive electrode, which has a satisfactory red response.

It is a further object of my invention to provide a photoconductive camera pickup tube having a good red spectral response.

It is another object of my invention to provide a target for a photoconductive pickup tube having a low dark current and a minimum time lag.

It is another object of my invention to provide a satisfactory photoconductive electrode formed of red sensitive photoconductive material.

It is a further object of my invention to provide a pickup tube target of antimony trisulfide having sumcient resistivity in the dark and satisfactory capacity for pickup tube operation.

In accordance with my invention I provide a photoconductive target electrode formed of a layer of selenium in contact with a layer of a red sensitive photoconductive material between the selenium film and the signal plate of the target. The red responsive photoconductive material may be antimony trisulfide or a film of cadmium selenide. By varying the thickness of the red sensitive film, the spectral response of the photoconductive target can be varied from an all-red response to a red-and blue-green response.

While certain specific embodiments have been illustrated and described, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

Figure l is a sectional view of a photoconductive pickup tube in accordance with my invention. Figure 2 is a sectional view of the target of the tube of Figure 1.

Figure 1 discloses a photoconductive pickup tube having an evacuated envelope Ml, within which is positioned an electron gun structure it and a target electrode I l. The electron gun It provides an electron beam l5 which is scanned over the surface of the target I l. The electron gun is of conventional design and consists essentially of a cathode electrode l6, a control grid I8 and accelerating electrodes and 2d. Electrodes 20 and 24 also provide partial focusing of beam [5. Cathode I6 is essentially a tubular electrode closed at one end facing the target electrode M. The closed cathode end is coated with thermionic emitting material, which is heated preferably by a non-inductive heater coil 26 to provide an electron emission. This emission is formed, in a well known manner, by elec trodes l8 and 20, into the electron beam 55.

The electrons of beam l5 are magnetically fo cused to a well defined point on the surface of target electrode M by a magnetic coil 23, which encloses the tube envelope it), as is shown. Structure 29 represents a pair of oppositely disposed coils connected in series to a source of direct current potential and rotatably mounted on the tubular envelope I0. These coils (not shown) produce a field transverse to the axis of the field of coil 28. Rotation of coil structure 29 around the tube envelope eliminates helical motion of the beam electrons entering the longitudinal field of coil 28, which helical motion is due to the unavoidable misalignment of the gun l2 during tube assembly. Details of the alignment structure 29 does not constitute a part of this invention but is set forth in greater detail in U. S. Patent 2,407,905 of Albert Rose. A yoke structure, indicated at 30, comprises essentially two pairs of coils, with the coils of each pair connected in series respectively to sources 3! and 33 of sawtooth currents, for providing line and frame scansion of beam i5 over the surface of target M. Such a deflection system is well known, and does not constitute a part of this invention. During tube operation, voltages are applied to the several electrodes as is indicated. These voltages represent appropriate Values for tube operation. However, operation of the tube need not be limited to these values.

Accelerating and focusing electrode 2d may comprise a conductive coating on the inner wall of the tube envelope ill. The conductive coating 24 extends to a point closely adjacent to target electrode l-l. Conductively connected to coating 24 by a lead 25, for example is a fine mesh screen 32, which is mounted in the tube envelope across the opening of a ring 36 sealed in the tube envelope ill.

Target electrode It comprises essentially a supporting insulating transparent plate 36, such as glass, for example, which in the tube of Figure 1 is a fiat end wall portion or face plate of envelope Hi. The target supporting plate may also consist of aseparate plate mounted within envelope IE9 and closely spaced from the tube end 36. The glass support 36 is coated on its surface facing the electron gun i2 with a transparent conductive film or signal plate 42. Such a conductive film may be formed from evaporated metal or of such material as stannic oxide to provide a conductive film. Conductive film 42 is coated with thin layers Mi and 45 of photoconductive ma terial.

During tube operation, appropriate voltages are applied to the several electrodes within the tube. The electron beam i5, which is formed thereby, is scanned over the surface of the photo-conductive material 55. Beam l5 approaches the surface of target i l at substantially zero, or gun cathode, potential. Electrons are deposited on the insulating surface of photoconductive coating I35, and drive it down toward an equilibrium potential, which approaches cathode or ground potential. The conductive signal plate 42 is connected by a lead 46 through a resistance 48 to a source of potential 50, to establish on signal plate 2 a potential from 10 to 20 volts positive, relative to the equilibrium potential of coating -55. When light is focused upon the target M, the illuminated portions of the target become conductive and because of the potential difference across films A l and 45, a current flows in these areas between the positive signal plate 42 and the scanned surface of film 45. This flow of current discharges the scanned surface of coating 45 towards the potential of signal plate 42. As the scanning beam it passes over the discharged areas of the screen, it tends to rapidly drive these areas toward equilibrium or ground potential. This almost instantaneous charging of a discharged area of the target toward the equilibrium potential, provides corresponding potential charges across resistance 48, as signal plate 42 is capacitively coupled to the scanned surface of film Q5. The changes in potential across resistance 48 are picked up and amplified, in a well known manner, to provide the video or output signal of the pick-up tube.

Antimony trisulfide film put down by evaporation in vacuum, forms a photoconductive target material having a good light response in the red region of the spectrum. However, the use of antimony trisulfide alone, as a photoconductive electrode in a camera pickup tube, results in several disadvantages. Although the material has satisfactory red response, it also possesses the disadvantage of having an inherently slow photoconductive response. Antimony trisulfide also possesses an unsatisfactory high current in the .dark. Furthermore, to provide sufiicient sensitivity for the tube, it is necessary to form the antimony trisulfide film with a thickness in the order of 0.1 mil. A target film as thin as this possesses too high an electrical capacity between its surfaces, so that the charges established on the scanned surface of the target cannot be erased instantly by a single scansion of the beam. This results in the charge and the picture on the viewing tube remaining longer than desired and producing the effect of an additional undesirable time lag in the photoconductive target.

In accordance with my invention, I have found that, if the target signal plate is first covered with a film of antimony trisulfide and then with a normally insulating film of amorphous selenium, a photoconducting target is obtained having the satisfactory red response of the antimony trisulfiide and the characteristic desired and necessary for satisfactory use as target material in a photoconductive pickup tube.

Figure 2 shows an enlarged view of the target of Figure 1, formed in accordance with my invention. Conductive film 42 desired manner, well known to those skilled in the art. On the exposed surface of the conductive film is first evaporated the thin film M of antimony trisulfide having a thickness lying within an optimum range between 0.1 micron and 0.5 micron. This antimony trisulfide film is formed by evaporating the material at around 450 C. in a vacuum. The desired thickness of the film 44 is determined by the color of the antimony trisulfide as it is deposited on the target The film starts with a bright yellow color and changes to yellowish brown then to a deep red. However, when the color is a yellowish brown, the evaporation is stopped, as experience has showed that a film of this color lies within the optimum range of thickness.

On the surface of the antimony trisulfide film 44 is formed the normally insulating layer 45 of amorphous selenium by evaporation in vacuum and in a manner discussed in my above cited copending application. The layer of selenium is formed with a thickness substantially of 2 to 5 microns. This thickness is determined by observing the selenium formation, during its evaporation, through a grating and noting the number of interference bands. described, has good red response and satisfactory dark resistivity. Furthermore, the target light absorbed in this layer produces any appreciable photoconductivity.

The present theory of photoconductivity is one, which pictures the energy of an impinging light beam as being absorbed within the photoconductive material and/or to provide holes,

to provide free electrons,

or positive charge car- A target made, as

61 riers. If a strong electric field is applied to the photoconductive material, the electrons in the conduction band will tend to move in the positive direction, while the holes or positive charge carriers tend to move in the negative direction.

Amorphous selenium is a material in which the holes have a long range when an electric field is applied across the material. In addition, selenium has a very high absorption coefficient for blue light. Thus when blue light falls on a selenium target the light does not penetrate com pletely through the selenium, but the holes, which are formed by the light, will themselves pass through the target toward the positive surface.

The character of amorphous selenium is such that there is little or no electron conductivity through the material when an electric field is applied across the material. It appears that the electrons released by the absorbed light are effectively trapped before they can move any great distance toward the positively charged surface of the material.

Antimony trisulfide is a material having a short range of carriers. Thus the blue light which is absorbed in the top layers of film 44, may establish charge carriers. But these carriers do not have sufficient range to reach the opposite surface, unless the film is very thin. however, a layer of selenium is deposited on top of an antimony sulfide film in the order of 0.1 to 0.5 micron, as described above, holes released will pass across the antimony trirelative to the scanned surface of the selenium film, these charge carriers or holes, in the antimony trisulfide will pass across the interface between the two films. since selenium ance to the pattern the photoconductive It is to be noted operation,

which selenium possesses under certain conditions. Thus, the charge carriers released by light striking the antimony trisulfide film can be conducted or transferred through the selenium film to the scanned target surface. The function of the selenium film enables to use of a thin antimony sulfide film in the material. obtained a red responsive photoconductive target without the disadvantage of time lag, high dark and the charge established on the signal plate.

If, the antimony trisulfide film is made 45 to the scanned provided a blue response, as swell as, red response for the photoconductive target. The double layer target, of Figure 2, is one which can either provide a red response or a red and blue response depending upon the relative thickness of the photoconductive film 44.

The target of the type described need not be limited to an antimony trisulfide film, as other red responsive photoconductive materials can be used. Such a material is cadmium selenide, which may be put down on the target plate in a manner described in the copending application of S. V. Forgue, Serial No. 198,130, filed November 29, 1950. Also, a red sensitive photoconductive layer of metallic selenium may be used in place of the antimony trisulfide film. The metallic selenium film is formed by evaporating first a thin layer of amorphous selenium on the conductive film 42 of the target I4, and then baking the target at a little over 100 C. in air to convert the amorphous selenium film to metallic selenium. The thicker layer of amorphous selenium is then put down on the metallic selenium film, in the manner described above.

The choice of the material which, when placed in contact with the selenium, will give an enhanced red response has so far been determined experimentally. A very thin layer of cadmium selenide is found to improve the red response for a positive polarity signal plate (hole conduction), provided that the cadmium selenide is baked in oxygen prior to the deposition of the amorphous selenium layer. On the other hand, a thin layer of cadmium selenide gave an enhanced red response for a negative polarity signal plate (electron conduction), provided that the cadmium selenide was not baked in oxygen after its deposition. These results are consistent with the above mentioned theory that the enhanced red response results from the transfer and conduction of free electrons or holes from the intermediate layer into the amorphous selenium film and across the selenium film to store a red signal on the scanned target surface.

What is claimed is:

l. A photoconductive target electrode for a pickup tube, said electrode comprising, a conductive film, a thin film of photoconductive material on one surface of said conductive film and a normally insulating film on said film of photoconductive material, said normally insulating film being of a material having the property of possessing long range charge carriers.

2. A photoconductive target electrode for a pickup tube, said target electrode comprising a transparent conductive film, a thin film of photoconductive material on one surface of said conductive film, and a coating of amorphous selenium on said film of photoconductive material.

3. A photoconductive target electrode for a pickup tube, said electrode comprising, a transparent support, a thin conductive film on one surface of said support plate, a thin film of photoconductive antimony trisulfide on the exposed face of said conductive film, and an amorphous selenium coating on said film of antimony trisulfide.

4. A photoconductive target electrode for a pickup tube, said target electrode comprising, a transparent support plate, a thin transparent conductive film on one surface of said support plate, a second film of antimony trisulfide coating said conductive film and an amorphous selenium coating on the surface of said antimony trisulfide film, said antimony trisulfide film having a thickness on the order of 1000 to 5000 angstrom units.

5. An electron discharge device comprising, an electron gun means for producing an electron beam along a. path, a target electrode mounted transversely to said beam path, said target electrode including a conductive film, a thin film of photoconductive material on one surface of said conductive film, and a normally insulating film on said photoconductive film, said normally insulating film being of a material having the property of possessing long range charge carriers.

6. A camera pickup tube comprising, an electron gun means for producing an electron beam along a path, a target electrode mounted transversely to said beam path, said target electrode including a conductive electrode, a thin film of photoconductive material on one surface of said conductive electrode, and a normally insulating film on the surface of said photoconductive film, said photoconductive material being of red light responsive material selected from the group consisting of antimony trisulfide, cadmium selenide, and metallic selenium, said normally insulating film having the property of possessing long range charge carriers.

'7. A pickup camera tube comprising, an electron gun means including a source of electrons for providing an electron beam along a path, a target electrode spaced from said gun means and arranged transversely to said beam path, said target electrode comprising, a transparent support plate, a thin conductive film on one surface of said support plate, a thin film of photoconductive antimony trisulfide on the exposed face of said conductive film, and an amorphous selenium coating on the surface of said film of antimony trisulfide.

8. A ickup camera tube comprising, an electron gun means including a source of electrons for providing an electron beam along a path, a target electrode spaced from said gun means and arranged transversely to said beam path, said target electrode including a transparent support plate, a thin conductive film on one surface of said support plate, a thin film of photoconductive cadmium selenide on the exposed face of said conductive film, and an amorphous selenium coating on the surface of said cadmium selenide film.

9. A pickup camera tube comprising, an electron gun means including a source of electrons for providing an electron beam along a path, a target electrode spaced from said gun means and arranged transversely to said beam path, said target electrode comprising, a transparent support plate, a thin conductive film on one surface of said support plate, a thin film of photoconductive metallic selenium on the exposed face of said conductive film, and an amorphous selenium coating on the surface of said metallic selenium film.

10. A photosensitive electrode comprising, a transparent support member, a transparent conductive film on one surface of said support member, a thin film of photoconductive material on the surface of said conductive film, and a film of normally insulating material on the surface of said photoconductive film, said normally insulating material having the property of possessing long range charge carriers.

11. A photosensitive electrode comprising, a transparent support, a thin transparent conductive film on one surface of said support, a thin film of photoconductive material on the surface of said conductive film, and a film of normally insulating material on the surface of said photonormally of posses insulating material having the property sing long range charge carriers.

References Cited in the file of this patent Number UNITED STATES PATENTS Name Date De Boer et a1 Oct. 6, 1936 Lubszynski et a1. Oct. 7, 1941 Iams June 30, 1942 Sheldon Oct. 17, 1950 

